Session C15 - Condensed Phase Dynamics: Clusters and Ionic Solutions.
FOCUS session, Monday morning, March 12
Room 211, Washington State Convention Center

[C15.001] Water Nanodroplets as a Reaction Medium: FT-ICR Studies of the Stability, Structure and Reactivity of Hydrated Ions and Ionic Water Clusters

With the help of a versatile ion source coupling laser vaporization with supersonic expansion, ionic clusters of the type X^±(H_2O)_n are easily generated, and if desired, they can be mass selected in a Fourier Transform Ion Cyclotron (FT-ICR) mass spectrometer. The central ion, X^± can be for instance H^+ or OH^-, a free electron, or an ionized metal such as Na^+, Ag^+, Mg^+, or Al^+. Such "nanodroplets" solvated with up to 200 molecules of water or other ligands slowly fragment in the collision-free environment of the FT-ICR trap. They lose in a controlled way the solvent molecules, one by one on a millisecond timescale. The products of reactions which occur in the nanodroplet as a result of the loss of the stabilizing ligand can in the high-resolution mass spectrometer be unambiguously identified. In this way, a variety of solution processes such as ionic dissolution, fragmentation, neutralization, precipitation, reduction-oxidation reactions, or acid-base catalyzed reactions can be investigated in molecular, microscopic detail. Small droplets and particles are important for a variety of atmospheric processes and reactions occurring both in the troposphere and the stratosphere. This suggests the possibility of preparing such nano-droplets of suitable composition, and using them as a model system for investigating a large variety of reactions important for atmospheric chemistry. In the present talk, we will describe our apparatus and external source, and discuss a variety of results obtained recently with it in our laboratory. The aldol condensation of acetaldehyde as an example of an acid-base catalyzed reaction and the precipitation of AgCl show that a number of well-known reactions in solution have their counterpart on a single molecule level in the cluster. The competition between electron detachment and water loss of hydrated electrons e^-(H_2O)_n, n=13-36, provides interesting and unexpected insights into the coupling dynamics of the electron to its water environment.

[C15.002] Controlling nanodroplet nucleation and growth in supersonic expansions

Nanodroplet aerosols form in supersonic expansions of condensible vapors. In conventional continuously expanding nozzles, aerosol nucleation and growth occur simultaneously over a wide range of temperatures and supersaturations. To better understand the physics of nanodroplet formation one needs to decouple nucleation and growth and estimate the supersaturation and temperature of nucleation. Here we present the first results of our efforts to decouple the two processes using custom nozzle shapes. Nanodroplet condensation in nozzles is usually studied by measuring pressure traces. These experiments only yield the conditions at the onset of condensation and little about the aerosol size distribution. Small angle neutron scattering (SANS) on aerosols, pioneered in our laboratory, permits in situ measurement of nanodroplet size distribution. Combining the thermodynamic state measurements, SANS, and light scattering with careful nozzle design allows us to find conditions under which nucleation can be separated from subsequent droplet growth. Under these conditions we observe fewer but considerably larger and more monodisperse particles than under similar conditions in conventional nozzles. Analysis of our results and our first estimates of nucleation rate deepen our understanding of aerosol formation under highly supersaturated conditions.

[C15.003] Dynamical Nucleation Theory: Sensitivity Analysis of the Intermolecular Potential

Vapor to liquid nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation rates. Since the vapor is comprised essentially of monomers, the formation of clusters is governed by monomer association and dissociation reactions. The formation of a cluster is impeded by a free energy of activation which, since no potential energy barrier exists, is entropic in nature. Variational transition state theory (VTST) provides a framework in which evaporation and condensation rate constants can be determined. The nucleation rate is then be obtained by solving the pseudo-first order kinetic equations. The rate constants governing the multi-step kinetics of small water cluster nucleation will be presented. In addition, an analysis was undertaken to explore what effect the uncertainties in the intermolecular potential have on the nucleation rate. The results of these studies and which directions future work should take will be discussed.

[C15.004] New Theoretical Insights for Excited State Proton Transfer Reactions in Solution

This abstract not available.

[C15.005] Coffee Break

This abstract not available.

[C15.006] Molecular Dynamics Simulations of Aqueous Halide Solvation in Cluster and Bulk Interfaces

This abstract not available.

[C15.007] 2D ^31P NMR Study of Takagi Group Diffusion in Rb_0.50(ND_4)_0.50D_2PO_4 Deuteron Glass

Slater proposed that KH_2PO_4 has 2 polar and 4 nonpolar (with higher energy E_b) H_2PO_4 H-bond configurations. Takagi proposed H_3PO_4 and HPO_4 groups with still higher energy E_a. Via intra bond H transfer, Takagi pairs form, diffuse independently, and annihilate, allowing H-bond reconfiguration. Our 2D ^31P chemical shift study gives the first direct evidence for Takagi group diffusion. A diffusion path past a D_2PO_4 group reverses its D-bonds, which may change it from a polar to a nonpolar group or vice versa. Polar and nonpolar group chemical shifts differ, giving a 3-peak 1D NMR spectrum at 50 K. 2D exchange NMR shows that Takagi group diffusion symmetrizes the D-bonds in about 15 s at 45 K. Temperature-dependent results indicate creation energy E_a=81 meV, and diffusion step energy E_b=12 meV in a fractal energy landscape.

Part C of program listing